The polyphenylene oxide resins are a well known family of engineering thermoplastics that are well known to the polymer art. These polymers may be made by a variety of catalytic and non-catalytic processes from the corresponding phenols or reactive derivatives thereof. By way of illustration, certain of the polyphenylene oxides are disclosed in Hay, U.S. Pat. Nos. 3,306,874 and 3,306,875, and in Stamatoff, U.S. Pat. Nos. 3,257,357 and 3,257,358. In the Hay patents, the polyphenylene oxides are prepared by an oxidative coupling reaction comprising passing an oxygen-containing gas through a reaction solution of a phenol and a metal-amine complex catalyst. Other disclosures relating to processes for preparing polyphenylene oxide resins, including graft copolymers of polyphenylene oxides with styrene type compounds, are found in Fox, U.S. Pat. No. 3,356,761; Sumitomo, U.K. No. 1,291,609; Bussink et al, U.S. Pat. Nos. 3,337,499; Blanchard et al, 3,219,626; Laakso et al, 3,342,892; Borman, 3,344,116; Hori et al, 3,384,619; Faurote et al, 3,440,219; Olander 3,956,242; and disclosures relating to metal based catalysts which do not include amines, are known from patents such as Wieden et al, U.S. Pat. Nos. 3,442,885 (copper-amidines); Nakashio et al, 3,573,257 (metal-alcoholate or-phenolate); Kobayashi et al, 3,455,880 (cobalt chelates); and the like. In the Stamatoff patents, the polyphenylene oxides are produced by reacting the corresponding phenolate ion with an initiator, such as peroxy acid salt, an acid peroxide, a hypohalite, and the like, in the presence of a complexing agent. Disclosures relating to non-catalytic processes, such as oxidation with lead dioxide, silver oxide, etc., are described in Price et al, U.S. Pat. Nos. 3,382,212. Cizek, 3,383,435 discloses polyphenylene oxide styrene resin compositions. All of the above-mentioned disclosures are incorporated by reference.
One of the outstanding properties of the polyphenylene oxide resins is the property of high-heat resistance which makes possible the use of this resin in applications where many thermoplastics would fail. This exceptional property, while desirable in a finished article, causes problems when melt blending is used to form compositions that include the polyphenylene ether resins. One problem is related to the fact that the high temperatures required for the processing of the polyphenylene ethers approach the temperatures at which additives begin to degrade. This problem is especially serious when compositions are formulated which include chemical foaming agents that are added to make a foamable thermoplastic composition as the temperatures at which the polyphenylene oxides are processed are sufficient to activate most thermolabile blowing agents. In addition, the decomposition products that result from the break down of the chemical blowing agent may be incompatible with the polyphenylene oxide polymer and may adversly affect the chemical stability and the physical properties of these compositions.
The applicants have discovered that foamable polyphenylene oxide compositions may be prepared by the use of a thermally unstable copolymer of a vinyl aromatic resin and a monoalkyl ester of an unsaturated dicarboxylic acid. It has been found that this type of a foaming agent may be extrusion blended at temperatures of about 450.degree. F. without inducing thermal decomposition. Injection molding at about 540.degree.-560.degree. F. is sufficient to cause decomposition of the foaming agent which expands the polyphenylene oxide composition. In addition it has also been found that these foaming agents may be coated onto pelletized polyphenylene oxide compositions and the resulting compositions may be foamed directly without having to extrusion blend the foamable composition.
Mizutani et al, J. Appl. Polym. Sci. Vol. 17, pp 3651-3660 (1973) describe a foaming agent which is a fine powdery crosslinked terpolymer of styrene, divinylbenzene and a monoalkylmaleate. This foaming agent is reported as having a decomposition temperature which results in foaming when the composition is extruded at 165.degree.-175.degree. C. This reference only mentions crosslinked terpolymers and does not suggest that an uncrosslinked binary copolymer, may be employed as a foaming agent for polyphenylene oxides.
Accordingly, it is a primary object of this invention to provide novel foamable compositions of polyphenylene oxide resins that have a sufficiently high thermal stability which will permit extrusion blending without thermal decomposition of the foaming agent.
It is also an object of this invention to provide a foamable polyphenylene oxide composition without extrusion blending the foaming agent with the resinous components of the composition.